A number of polyolefin-based hydrophobic synthetic fibres are known, for example hydrophobic textile fibres with dirt and stain resistant properties. However, such fibres generally contain cationic antistatic agents that are undesirable or unsuitable for personal hygiene and medical products for toxicological reasons, since they often exhibit skin irritating properties due to their low pH. Also, some components may during use release di- or tri-ethanolamine, which is suspected of causing allergic reactions. It has previously proved difficult to produce fibres for hygienic or medical use having good cardability properties together with satisfactory hydrophobic properties. This is particularly important for the many applications in which it is desired that hydrophobic fibres may be carded using high carding speeds.
Hygienic products such as disposible diapers, sanitary napkins and adult incontinence pads generally have barriers through which fluids absorbed by the absorbent core are not able to penetrate, e.g. in the form of side guards, other structural elements, or as back sheet material opposite to the skin. Such barriers may comprise a nonwoven material prepared from hydrophobic staple fibres or a spunbonded material prepared directly from a hydrophobic polymer. However, spunbonded materials are very flat and film-like, and do not have the soft, uniform, textile-like comfort that one finds in nonwovens. Spunbonded fabrics are therefore not the optimal choice for liquid barriers designed to be in contact with the skin of the user. Also, spunbonded nonwovens have a non-uniform distribution of fibres, which results in weak areas (holes) that limit the liquid barrier properties of the fabrics, so that web uniformity becomes the limiting factor for the hydrophobic characteristics. As for nonwovens prepared from staple fibres, these tend not to be sufficiently hydrophobic for such liquid barriers, due to the fact that during the spinning process, the fibres are treated with a "spin finish" which facilitates the spinning process by lubricating the fibres and making them antistatic. However, as a result of the spin finish treatment, in particular the use of an antistatic agent, which by nature is more or less hydrophilic, the fibres become somewhat hydrophilic, which in the present context is undesirable. On the other hand, fibres with the desired degree of hydrophobicity have generally had suboptimum antistatic properties.
EP 0 557 024 A1 describes polyolefin fibres treated with an antistatic agent which is a neutralized phosphate salt, and optionally with a hydrophobic lubricant selected from mineral oils, paraffinic waxes, polyglycols and silicones, the fibres having an hydrostatic head value of at least 102 mm. WO 94/20664 describes a method for producing cardable, hydrophobic polyolefin-based staple fibres using two spin finishes, in which the second spin finish is a dispersion comprising an antistatic agent, preferably an anionic or non-ionic antistatic agent, and, as a hydrophobic agent, a natural or synthetic hydrocarbon wax or wax mixture, and optionally a silicone compound.
The present invention represents a different and highly effective approach to the problem of providing polyolefin staple fibres with an optimum combination of hydrophobic and antistatic properties, thereby making them suitable for the production, in particular by means of high-speed carding, of nonwovens with optimum strength and hydrophobic characteristics. Furthermore, the invention is based on the use of substances which are not irritating to the skin.
An object of the present invention is therefore to provide hydrophobic thermobondable synthetic fibres, in particular for hygienic applications, with both optimum hydrophobic and antistatic properties, and thus with improved carding properties suitable for preparation of nonwovens showing superior strength. A further object of the present invention is to improve the application and distribution of spin finish on the fibres, thus improving fibre uniformity, allowing increased carding speed and improved web uniformity in the carding process, which in turn results in nonwovens with improved hydrophobic properties.